The synthesis of MSA from methyl mercaptan and chlorine according to the reaction: EQU CH.sub.3 SH+3 Cl.sub.2 +3 H.sub.2 O.fwdarw.CH.sub.3 SO.sub.3 H+6 HCl
produces a large amount of hydrochloric acid by-product. The HCl gas flow separates from the reaction medium, entraining some of the volatile compounds, and this entrainment is proportionately larger when the reaction is carried out at 100.degree. C. By means known per se, the chlorine and the methyl mercaptan are readily eliminated from the hydrochloric acid. The water is not a hindrance since the aim is to recover a 33% solution of HCl in water.
However, during the reaction which leads to methanesulphonic acid, a relatively volatile intermediate is produced, methylsulphonyl chloride CH.sub.3 SO.sub.2 Cl (MSC), which, on working up the gaseous HCl with water, converts into methanesulphonic acid, thereby contaminating the hydrochloric solution and making it unsuitable for a certain number of applications.
The entrainment of MSC is considerable since the contact between the HCl and the MSC takes place at high temperature. The excess MSC is readily removed using the known standard techniques such as, for example washing the hydrochloric effluent with the MSA produced in the reaction and preferably purified. Neverless, appreciable amounts of MSC are entrained by the HCl since the operations do not take place at low temperature, but in the region of room temperature. It is known that depending on the temperature (see Table I), the MSC content of the hydrochloric acid takes increasing values.
TABLE I Temperature MSC in HCl gas MSA in aqueous (.degree. C.) (ppm) 33% HCl (ppm) -5 728 203 0 1245 350 5 1770 495 8 2570 720 12 4610 1290 15 5440 1520 18 7120 1990 21 8830 2470
In a usual process for the synthesis of MSA, the HCl flow is at a temperature in the region of 20.degree. C. It thus contains from 5000 to 9000 ppm of MSC, which lead to a 33% hydrochloric solution containing from 1500 to 2500 ppm of MSA.